1. Field of the Invention
This invention relates generally to a system and method for providing communications signals between airborne platforms and spacecraft that are protected from jamming signals and, more particularly, to a system and method for providing communications signals between aircraft and spacecraft, where the signals operate at or around 60 GHz, and where the aircraft is flying above 40,000 feet so that the signals transmitted from the aircraft to the spacecraft are prevented from being jammed by a jamming signal originating from the ground and the signals from both the aircraft and the spacecraft are prevented from being intercepted from the ground.
2. Discussion of the Related Art
Many applications, both commercial and military, use communications signals transmitted between a spacecraft, such as an earth orbiting satellite, and an aircraft or ground station. For example, high altitude long endurance (HALE) aircraft, such as several of the military's unmanned aerial vehicles (UAV), typically communicate using transponder satellite channels provided on various spacecraft using the Ka frequency band (20-30 GHz), Ku frequency band (12-15 GHz), L frequency band (1-2 GHz) and other similar frequency bands. These frequency bands are typically selected because they are often internationally allocated bands for satellite communications and they allow relatively small antennas because of their relatively short wavelengths.
Various UAVs operate with different altitudes, ranges and capabilities. While tactical UAVs nominally operate within line-of-sight of their launch and control stations, strategic reconnaissance and strike platforms typically operate beyond line-of-sight and rely on relay links to maintain mission connectivity. In addition to organic intelligence, data gathering and strike roles, UAVs are now being pressed into relay roles that extend communications to greater distances or beyond terrain blockages to traditional forces and even to other unmanned systems. Satellite communications signals provide tremendous reach and coverage for UAVs in areas denied to ground forces, but such communications often result in increased vulnerability, where the greater the distance or broader the communications area, the greater the potential exposure to counter measures, such as signal jamming.
Communication signals for these and other applications, typically military applications, are generally highly susceptible to jamming signals from the ground. Because the antennas used by a spacecraft to transmit signals to an aircraft or the ground typically have a very large footprint on the ground, sometimes on the order of hundreds of miles, unsophisticated transmitters that transmit at the frequency of the satellite signals can be placed anywhere in that footprint, and when directed towards the satellite can prevent the satellite from acceptably receiving signals from the aircraft or the ground if the intensity of jamming signal is large enough. In other words, because the signals from the aircraft are transmitted upwards towards the spacecraft, a jamming transmitter on the ground that would be behind the aircraft and in the footprint of the satellite antenna could prevent the spacecraft from receiving the signals from the aircraft. This occurs because the spacecraft is unable to distinguish the aircraft from other transmission points on the ground.
Various techniques are known in the art for minimizing or eliminating jamming signals. However, such techniques often have undesirable consequences and limitations. For example, it is known to provide frequency hopping or spread spectrum transmitters that change their transmission frequency in an attempt to operate at frequencies other than the frequency of the jamming signal. However, the jam-resistance of these spread spectrum techniques is inversely related to the bandwidth of the signal, where the wider the transmission bandwidth, the less effective the spectrum spreading becomes. Further, as communications between aircraft and spacecraft require more bandwidth, possibly on the order of 100 mega bits per second, the limitations of spread spectrum transmitters increases. It is also known in the art to use nulling antennas where the antenna gain pattern of the transmitter is designed to have a null (no gain) in the direction that the jamming signal may be originating from. However, if the intended transmitter of signals to the spacecraft is proximate to or in-line with the jamming location, then it will also not receive the transmitted signal.
Furthermore, signals transmitted from either the aircraft or the spacecraft are subject to detection by sensitive equipment on the ground. Intercept of these signals may permit evaluation of aircraft or spacecraft operations and potentially expose sensitive information.
It is known in the art that signals at or around 60 GHz are readily absorbed in the atmosphere. Particularly, oxygen molecules (O2) in the air have electron orbits including resonant frequencies at or around 60 GHz that cause radiation at 60 GHz to be readily absorbed in the atmosphere. This absorption is so great that very high powered signals in this frequency band are only able to travel a short distance through the air. Thus, a spacecraft that is operable to receive signals at 60 GHz will not see such transmission signals from the ground because those signals will be absorbed by the atmosphere well before they reach outer space.
Based on this phenomenon, it is also known in the art to use the 60 GHz frequency band for communications signals transmitted between spacecraft for various applications because those signals will not be absorbed where the atmosphere is not present. Thus, these signals transmitted in space are not susceptible to jamming signals from the ground or are not intercepted by ground stations. Thus, although the electronics necessary to transmit and receive signals at this high of a frequency band are typically complex, such systems are sometimes desirable because of their anti-jamming capabilities.